1. Field of the Invention
This invention is in the field of microwave energy transmission and particularly relates to microwave harmonic absorption filters.
2. Description of the Prior Art
In many microwave signal processing systems, it is required and/or desirable to transmit microwave energy in one range of frequencies called the pass-band while attenuating energy in a second range of frequencies called the stop-band. This can be accomplished by microwave filters which fall into many categories according to the relationship between their pass-band and stop-band. As and example, a low pass filter has a pass-band situated at a lower range of frequencies than its stop-band, while a high pass filter has its pass-band at higher frequencies than its stop-band.
It is also possible to subdivide low pass filters into two groups: reflective low pass filters and absorptive low pass filters. The former type reflect back most of the incident energy for frequencies within the stop-band, while the latter do not reflect energy, but instead, absorb incident energy within the stop-band range of frequencies in an appropriate lossy medium. Absorptive low pass filters may have the additional property of absorbing incident energy for any propagating mode of the stop-band. Thus, they do not only absorb in the fundamental mode of propagation of energy, but also in the next few higher order modes.
One type of harmonic absorption filter which has been used is known as the "leaky-wall" type. In these "leaky-wall" harmonic absorption filters, a main waveguide is coupled to one or more secondary waveguides by means of apertures. The pass-band range of frequencies is transmitted in the main or primary waveguide whereas the stop-band range of frequencies is transmitted from the primary waveguide through apertures into the secondary waveguides wherein they are greatly attenuated. Several such "leaky-type" microwave harmonic absorption filters are described in the patent literature.
One such "leaky-type" harmonic absorption filter is disclosed by Farr et al. in U.S. Pat. No. 2,877,434. The Farr et al. filter comprises a cylindrical conductive wall forming a waveguide for transmitting electromagnetic waves in first and second modes that tend to produce first transversely directed and second longitudinally directed currents with the cylindrical conductive wall. This filter also has a mode absorber comprising an annular chamber of rectangular cross-section disposed contiguously on the outside of the cylindrical wall with at least one longitudinally disposed slot in the wall leading to the annular chamber and an annular sheet of resistive material in the chamber.
Another microwave harmonic absorption filter is disclosed by Wantuch in U.S. Pat. No. 3,187,277. In the Wantuch filter, a main waveguide channel is provided with a series of apertures on all four sides and each aperture is provided with an associated short waveguiding passageway. Subordinate microwave chambers containing lossy material extend parallel to the main waveguide and many of the small waveguiding passageways are coupled to a single one of these subordinate lossy chambers. Dielectric rods may extend through the passageways to more strongly couple harmonics from the main waveguide to the subordinate waveguide chambers where the harmonic energy is dissipated. By extending the rods physically into the main waveguide, or the lossy subordinate chambers, or both, it is stated that much greater attenuation of the harmonic energy is achieved.
The harmonic absorption filters heretofor available, however, have not achieved single mode pass-bands extending to one octave of frequency i.e., a ratio of 2/1. In fact, due to the properties of the rectangular waveguides used in many of the prior art filters, the ratio has typically only been 1.5/1.